I. Field of the Invention
The present invention relates generally to a method of delivering a stimulation pulse to a patient's heart utilizing a cardiac rhythm management device, and more particularly relates to a method of reducing the affects of intrinsic detection latency during an automatic capture and/or sensing threshold determination. The method of the present invention is initiated immediately prior to delivering a stimulation pulse to the heart tissue, wherein delivery of the stimulation is terminated or delayed if the amplitude of a portion of a sensed electrogram signal exceeds a predetermined threshold. Further, the method of the present invention minimizes the delivery of stimulation pulses proximate in time with an intrinsic event due to intrinsic detection latency (pseudo-fusion).
II. Discussion of the Related Art
Cardiac rhythm management devices have enjoyed widespread use and popularity throughout time as a means for supplanting some or all of the natural pacing functions of an abnormal heart. Among the various heart abnormalities remedied by cardiac rhythm management devices include total or partial heart block, bradycardia, tachycardia, myocardial infarctions, congestive heart failure, congenital heart disorders, and various other arrhythmias and rhythm disturbances within the heart. The fundamental components of a rhythm management device include an electronic pulse generator for generating stimulation pulses to the heart and an electrode arrangement for both delivering the stimulation pulses and sensing intrinsic and/or evoked events of the heart.
Conventional rhythm management devices may be designed to stimulate the ventricle, atrium or both the ventricles and atriums depending upon the heart abnormality. Regardless of the type of rhythm management device employed to restore the heart's natural rhythm, all operate to stimulate heart tissue cells adjacent to an electrode which is employed in the heart and electrically coupled to the rhythm management device. The delivery of the stimulation pulse is often dependent upon a preprogrammed timing sequence. Without limitation, the timing sequence may, for example, depend upon sensing or failure to sense an intrinsic event during a period that is measured from a previous sensed intrinsic event or prior stimulation. If certain predetermined requirements are not met during the timing sequence, then the rhythm management device may be preprogrammed to generate and deliver a stimulation pulse to the heart. When the stimulation evokes a response in the heart, this response is typically referred to as "capture" and is a function of the positive and negative charges found in each myocardial cell within the heart.
The success of the cardiac rhythm management device in depolarizing or "capturing" the heart hinges on whether the energy of the stimulation pulse as delivered to the myocardium exceeds a threshold value. This threshold value, referred to as the capture threshold, represents the amount of electrical energy required to alter the permeability of the myocardial cells to thereby initiate cell depolarization. If the energy of the stimulation pulse does not exceed the capture threshold, then the permeability of the myocardial cells will not be altered and thus no depolarization will result. If, on the other hand, the energy of the stimulation pulse exceeds the capture threshold, then the permeability of the myocardial cells will be altered such that depolarization will result. Changes in the capture threshold may be detected by monitoring the efficacy of stimulating pulses at various energy levels.
The ability to detect capture is extremely desirable in that delivering stimulation pulses having energy far in excess of the patient's capture threshold is wasteful of the rhythm management devices limited power supply. In order to minimize current drain on the power supply, it is desirable to automatically adjust the rhythm management device such that the amount of stimulation energy delivered to the myocardium is maintained at the lowest level that will reliably capture the heart. To accomplish this, a process known as "capture verification" must be performed wherein the rhythm management device monitors to determine whether an evoked depolarization or intrinsic event occurs in the heart following the delivery of each stimulus pulse.
At times, a stimulation pulse may be delivered coincidental to a depolarization by an intrinsic beat (hereinafter referred to as "fusion" or "a fusion beat"). From a surface ECG, the fusion beats manifest themselves by a pacing spike followed by an intrinsic QRS complex. Further, due to intrinsic detection latency, a stimulation pulse may be delivered after intrinsic activation has already begun (hereinafter referred to as pseudo-fusion). From a surface ECG, it is seen that the stimulation pulse falls inside the intrinsic QRS complex. The stimulation pulses may or may not capture the myocardium. During normal delivery of a stimulation pulse, fusion and/or pseudo-fusion beats may be of little consequence except some energy loss due to unnecessary pacing output. However, during autocapture or autothresholding, the impact of fusion or pseudo-fusion can be rather different.
During autocapture or autothreshold, fusion beats may be detected as capture for amplitude-based detection methods. Thus, even though the stimulation pulse may be below threshold, the evoked response detection remains positive. As a result, the threshold may be identified at a lower amount than the actual threshold. Pseudo-fusion may be detected either as capture or non-capture depending upon timing of the occurrence of pseudo-fusion. If a stimulation pulse is delivered at an earlier portion of the QRS complex, then the stimulation pulse is more likely to be detected as capture and the consequence is the same as a fusion beat. If pseudo-fusion is detected as non-capture, a backup pulse may be issued between the QRS complex and a T wave which is undesirable.
During automatic threshold determination, pseudo-fusion beats may cause false detection of either capture or non-capture. When pseudo-fusion is detected as capture, an error in threshold measurement may arise. In many instances, occurrence of pseudo-fusion is caused by the inherent latency of sensing an intrinsic event. This latency often results from a sensing threshold level that is normally higher than front portions of the QRS complex of the endocardial signals, which prevents a detection by the rhythm management device of the front portions of the QRS complex. Other factors that may contribute to latency in intrinsic detection include sensing channel phase delay. Thus, there is a need for a method that reduces unnecessary autothresholding, error in threshold measurement, and other undesirable affects of fusion and pseudo-fusion during capture verification and autothreshold determination. There is a further need for a method that manages the timing of delivery of backup stimulation that avoids stimulating during undesirable portions of a timing cycle.